The present invention relates to a two-cycle engine, especially as a drive engine in a portable, manually-guided tool or implement such as a power chain saw, a brush cutter, a trimmer, a cut-off machine, etc.
A two-cycle engine of this type is known from DE 199 00 445 A1. A combustion chamber formed in the cylinder is connected to the crankcase via transfer passages, the mixture required for combustion being conveyed to the crankcase. In order to ensure that as-little uncombusted fuel as possible is lost through the exhaust or outlet during the scavenging of the combustion chamber, the transfer passages close to the exhaust are connected to an air duct and fuel-free air is drawn in through the transfer passages during the intake stroke. The air is then held at the front of the transfer passages and enters first the next time the mixture transfers into the combustion chamber. The mixture flowing out of the crankcase follows some time later and the scavenging losses flowing out of the exhaust during the scavenging of the combustion chamber come largely from the forward positioned scavenging air.
In practice, a number of problems occur during the metering of the fuel required to operate the internal combustion engine by a carburetor. For example, at idle it is necessary to guarantee that the air duct is fully closed in order to prevent the idle mixture becoming too lean in an uncontrolled manner in the combustion chamber as a result of the air flowing into it. During acceleration, too, the opening of the air duct renders the mixture too lean as a result of which the speed of the internal combustion engine increases only reluctantly to the desired level.
On the other hand, it is important to guarantee that the air duct remains as free as possible from fuel at full throttle in order that the significant reduction in exhaust gas emissions which the forward positioned scavenging air is designed to achieve can be obtained.
The invention is based on the object of designing a two-cycle engine of the aforementioned type in such a manner that it is possible to reliably prevent the mixture in the combustion chamber from becoming too lean at idle and part throttle while retaining the advantageous effects of the supply of fuel-free air with which to scavenge the combustion chamber at full throttle.